This invention relates to the art of titanium forging and more specifically to a process for electrophoretically depositing components of a vitreous forging lubricant precoat on the surface of titanium and/or titanium alloy workpieces. Additionally, the invention relates to the control of the thickness of the forging lubricant precoat over the surface of the workpiece. The process of the invention is also useful in identifying defects in the surface of the workpieces which might otherwise go unnoticed.
Vitreous or glass-like forging lubricants for titanium alloys are known. Commonly, the forging lubricant is provided on the titanium workpiece by initially dipping, spraying, or painting a suspension of lubricant components as a precoat on the surface of the workpiece. At forging temperatures, the precoat becomes a molten glass having the approximate viscosity of bottled honey (about 40 PaS). The fused glass provides a thick film, hydrodynamic lubricant to facilitate the flow of the titanium. Glasses primarily comprised of borates, high-alkali silicates and borosilicates and phosphates have found commercial acceptance.
Vitreous coating formulations generally comprise one or more glass frits in a finely divided state suspended in an organic fluid such as isopropanol. Suspension aides such as clay and inert fillers are also typical of these compositions. Workpieces are generally coated by dipping, spraying or painting the suspension on the workpiece. In order to build up sufficient coating thickness, several applications are often necessary. Control of the thickness of the lubricant precoat over the surface of the workpiece is difficult using dipping, spraying or painting methods. Thickness control is essential in order that an acceptable surface finish may be provided on precision forgings. Specifically, gravity and often complex workpiece geometry work together to cause a thick coating to be developed in some portions of the workpiece, while other portions have only very thin coatings. Uneven coating may result in mottled or rippled portions on the final forged surface of the workpiece. If the coating is too thin, there may be localized contact between the forging die and the workpiece. Diffusion bonding and die wash may result.
In addition to the problems of uneven precoat thickness using the known methods for applying forging lubricant compositions, "green" (unfired) strength of such coatings is also inadequate. Specifically, titanium workpieces are often subject to considerable handling prior to forging which may result in the coating being chipped off or scored if the "green" strength is too low.
Electrophoretic processes have been suggested for applying the lubricant precoat. Such have been suggested because electrophoretic processes are known to result in uniform coating thickness. The process of electrophoresis involves the movement and deposition of discrete charged particles in a fluid suspension. Negatively charged particles are deposited on a positive electrode (anode) while positively charged particles are moved to and discharged or deposited on a negative electrode (cathode). Electrophoretic processes may be carried out in an aqueous or a solvent-based system.
Electrophoretic processes are used to deposit both organic and inorganic films on an appropriate electrode. A latex glove may be deposited by electrophoretic deposition of the latex from an emulsion onto an anodically charged metal form. Electropainting is an important electrophoretic process used for producing paint coatings on metal articles such as toys, furniture, bicycles, etc. Electrophoresis is particularly useful in coating automobile bodies due to its ability to relatively evenly coat interior and exterior surfaces as well as recesses and occluded areas.
Electrophoresis has long been used in inorganic processes such as the purifying of clay. Charged clay particles are easily separated from the overburden by the application of an electrical potential to a water suspension of a raw clay. More recently, electrophoresis has been used in the deposition of porcelain enamels on steel bodies for appliances. Since these coated articles are not normally handled to any great degree between the coating and the firing of the enamel, the fact that the coatings have minimal adhesion or green strength following the electrophoretic coating process presents little problem. U.S. Pat. No. 3,484,357 is illustrative of electrophoretic coating processes to deposit a porcelain enamel on steel.
Glass coatings have also been commercially applied by electrophoretic deposition for the manufacture of substrates for electronic circuitry. Again, low green strength properties do not present a substantial disadvantage. A similar process for depositing a vitreous insulation on wire for forming a dielectric layer on electrical condensers, resistor units, etc. as well as the enameling of cooking utensils is disclosed in U.S. Pat. No. 2,321,439.
Prior attempts to utilize these beneficial electrophoretic coating processes to deposit glass lubricant components on a titanium forging preform such as for a turbine fan blade have met with failure. A titanium alloy workpiece is quickly anodized to form titanium oxides on the surface when charged as an anode in typical electrophoretic coating solutions. An electrically insulating layer of titanium dioxide (TiO.sub.2) is quickly formed on the surface of the titanium workpiece. Such a high resistance layer reduces or essentially eliminates any possibility of electrophoretic deposition of a coating onto the surface of a titanium workpiece since the applied voltage is almost entirely lost to the high resistance coating of TiO.sub.2. Thus, electrophoretic processes which have worked well for deposition of glass layers on steel or other conductive substrates have not been capable of use for applying a glass layer to a titanium alloy substrate.
It is necessary, and often critical, that defects in a titanium workpiece be identified before the final product is released for use. Defects such as forging laps, cracks, crevices, compositional inhomogeneities, etc., often go undetected in the early stages of forging and are sometimes completely obscured by later forging steps. Inspection at the end of the forging process is often incapable of detecting these obscured defects which could result in the in-service failure of a forged article. In some applications, such failure could have disasterous consequences.
In an effort to avoid failure of a forged article in use due to an undetected defect, inspection at various forging stages is necessary. Thus, particularly in critical applications, a workpiece is inspected at several points in the forging operation. Sonic inspection or the so-called "blue etch anodize" inspection or a combination of both are common.
In the "blue etch anodize" process, the titanium workpiece is anodized to an overall blue color. Defects such as forging laps, cracks, crevices, etc., show up as an amber or a reddish purple area in the otherwise blue surface. Defective parts are, thus, detected and removed from further processing. For critical applications, each part must be so inspected after each stage in the forging process. Obviously, such multi-step individual handling and inspection greatly increases the cost of the final article.